Field of the Invention
The present invention relates to a multi-stage rolling mill having work rolls of a comparatively small diameter and, more particularly, to a roll bending apparatus for imparting a bend to the work rolls of small diameter in a multi-stage rolling mill.
In the fields of rolling mills, there is an increasing demand for saving of energy and reduction in the generation of edge drop. In particular, rolling mills for rolling web materials such as thin and wide sheets, as well as sheets of a high hardness, are required to provide a high degree of uniformity of thickness along the breadth of the sheet. In order to meet such demands, it has been proposed to reduce the diameter of the work rolls as much as possible, so as to enable a crown control, i.e., the control of deflection of the work rolls, by work roll bending apparatus which imparts a controlled bend to each work roll. The work roll bending apparatus is usually installed in a space between back-up roll chocks which support both ends of upper and lower back-up rolls which back up upper and lower work rolls, respectively. Thus, an extremely small diameter of the work rolls makes it difficult to find the space for the installation of the work roll bending apparatus, because in such a case the vertical distance between both back-up roll chocks is very small.
On the other hand, the reduced diameter of the work rolls lowers the flexural rigidity of the work roll, so that the work rolls tend to deflect in the horizontal directions, causing unfavourable effect on the cross-section of the rolled product. Thus, a reduction in the diameter of work rolls in turn requires both a reduction in the size of the work roll bending apparatus and a supporting mechanism which prevents horizontal deflection of the work rolls.
A prior art shown in the specification of the U.S. Pat. No. 4,369,646 has roll bending apparatus for work rolls and also roll bending apparatus for upper and lower intermediate rolls which back up the work rolls. These roll bending apparatus, however, cannot be used practically when the diameter of the work rolls is further decreased.
On the other hand, rolling mills are recently developed which have both the supporting mechanism for preventing horizontal deflection of the work rolls and roll bending apparatus for work rolls. A four-stage rolling mill as an example of such rolling mills is shown in U.S. Pat. No. 4,598,566. In this rolling mill, each of the bearing boxes for the work roll is supported in the following manner. Namely, each of the work roll chock which rotatably supports one end of a work roll is secured to a supporting member through a pair of guides which engage with both sides of the work roll chock. Thus, almost no play or gap is formed between each guide member and the work roll chock nor between each guide member and the supporting member. This imposes the following problem.
The work roll is progressively worn during long rolling operation, so that the work rolls are periodically replaced and the surfaces of the demounted work rolls are ground further use. In consequence, the diameter of the work roll is gradually decreased. On the other hand, in order to prevent undesirable horizontal deflection of the work roll, it is necessary that the support roll of the mechanism for preventing the horizontal deflection is held in contact with the work roll. However, when the work roll diameter has been decreased as a result of repeated grinding, it becomes materially impossible to keep the support roll in contact with the work roll, because no substantial play or gap exists in the structure which supports the work roll chock. In consequence, a slight gap is left between the work roll and the support roll, so that the work roll is deflected horizontally, with a result that the quality of the rolled product is impaired seriously.
The rolling mill disclosed in U.S. Pat. No. 4,598,566 incorporates a work roll bending apparatus which is adapted to impart the bending force to the work roll by means of a lever which is swingable about a fulcrum. The swinging motion of the lever is caused by a cylinder which is disposed on the same side of the fulcrum as the work roll. This roll bending apparatus has quite a complicated construction which can hardly be made compact.